Onvlak

Onvlak is a Dutch term used in engineering and metrology to describe a surface that does not lie in a single, ideal plane. It is the opposite of vlak, which refers to a perfect plane surface. In practice, onvlak denotes the deviation of a real surface from an imagined reference plane.

The main quantities used to characterize onvlak are flatness and the related concept of a tolerance zone. Flatness represents the maximum distance between points on the surface and a reference plane. The tolerance zone is a pair of parallel planes bounding the intended plane. In metrology, flatness can be specified independently of orientation and is assessed with instruments such as a coordinate measuring machine, a profilometer, or optical interferometers. Roughness and waviness describe smaller-scale texture and do not substitute for flatness.

Causes of onvlak include machining errors, tool deflection, thermal distortion, surface finishing, and warping in castings. Consequences of excessive onvlak can impair fit, sealing, or function in mechanical assemblies, and tolerance analysis must account for it in planning. Techniques to reduce onvlak include resurfacing or machining to bring the surface within the specified flatness tolerance, and the use of appropriate fixturing and controlled cooling during manufacture.

Etymology and usage: the term derives from Dutch on- (not) and vlak (plane). In practical contexts, onvlak is addressed through metrology practices to ensure components meet required geometric tolerances and assemble correctly.

See also: flatness, surface roughness, metrology, geometric dimensioning and tolerancing.